Stability and robustness of metal-supported SOFCs

Tubular metal-supported SOFCs with YSZ electrolyte and electrodes comprising porous YSZ backbone and infiltrated Ni and LSM catalysts are operated at 700 °C. Tolerance to five complete anode redox cycles and five very rapid thermal cycles is demonstrated. The power output of a cell with as-infiltrated Ni anode degrades rapidly over 15 h operation. This degradation can be attributed primarily to coarsening of the fine infiltrated Ni particles. A cell in which the infiltrated Ni anode is precoarsened at 800 °C before operation at 700 °C shows dramatically improved stability. Stable operation over 350 h is demonstrated.     

High performance solid oxide fuel cells based on tri-layer yttria-stabilized zirconia by low temperature sintering process

Performance of solid oxide fuel cells (SOFCs) depends critically on the composition and microstructure of the electrodes. It is fabricated a dense yttria-stabilized zirconia (YSZ) electrolyte layer sandwiched between two porous YSZ layers at low temperature. The advantages of this structure include excellent structural stability and unique flexibility for evaluation of new electrode materials for SOFC applications, which would be difficult or impossible to be evaluated using conventional cell fabrication techniques because of incompatibility with YSZ under processing conditions. The porosity of porous YSZ increases from 65.8% to 68.6% as the firing temperature decreased from 1350 to 1200 °C. The open cell voltages of the cells based on the tri-layers of YSZ, co-fired using a two-step sintering at 1200 °C, are above 1.0 V at 700-800 °C, and the peak power densities of cells infiltrated LSCF and Pd-SDC electrodes are about 525, 733, and 935 mW cm⁻² at 700, 750, and 800 °C, respectively.     

Behavior of 3 mol% yttria-stabilized tetragonal zirconia polycrystal film prepared by slurry spin coating

A dense and uniform 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3YSZ) electrolyte film of 6 μm in thickness was fabricated by slurry spin coating on a porous NiO/3YSZ anode substrate. Composite cathodes of La_0.7Sr_0.3MnO₃ impregnated with Sm_0.2Ce_0.8O_1.9 were fabricated on the 3YSZ films. A single cell produced in this way was tested at 700, 750 and 800 °C with hydrogen as fuel and stationary air as oxidant. Test results revealed an open-circuit voltage of 1.04 V at 800 °C, and maximum power density of 551, 895 and 1143 mW cm⁻² at 700, 750 and 800 °C, respectively. Impedance spectra results demonstrated that the cell performance was determined by the polarization resistance of the cathode.     

GdBaCo2O5+x layered perovskite as an intermediate temperature solid oxide fuel cell cathode

GdBaCo₂O_5+x (GBCO) was evaluated as a cathode for intermediate-temperature solid oxide fuel cells. A porous layer of GBCO was deposited on an anode-supported fuel cell consisting of a 15 μm thick electrolyte of yttria-stabilized zirconia (YSZ) prepared by dense screen-printing and a Ni–YSZ cermet as an anode (Ni–YSZ/YSZ/GBCO). Values of power density of 150 mW cm⁻² at 700 °C and ca. 250 mW cm⁻² at 800 °C are reported for this standard configuration using 5% of H₂ in nitrogen as fuel. An intermediate porous layer of YSZ was introduced between the electrolyte and the cathode improving the performance of the cell. Values for power density of 300 mW cm⁻² at 700 °C and ca. 500 mW cm⁻² at 800 °C in this configuration were achieved.     

Ni–YSZ-supported tubular solid oxide fuel cells with GDC interlayer between YSZ electrolyte and LSCF cathode

Highly sinterable gadolinia doped ceria (GDC) powders are prepared by carbonate coprecipitation and applied to the GDC interlayer in Ni–YSZ (yttria stabilized zirconia)-supported tubular solid oxide fuel cell in order to prevent the reaction between YSZ electrolyte and LSCF (La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ) cathode materials. The formation of highly resistive phase at the YSZ/LSCF interface was effectively blocked by the low-temperature densification of GDC interlayer using carbonate-derived active GDC powders and the suppression of Sr diffusion toward YSZ electrolyte via GDC interlayer by tuning the heat-treatment temperature for cathode fabrication. The power density of the cell with the configuration of Ni–YSZ/YSZ/GDC/LSCF–GDC/LSCF was as high as 906 mW cm⁻², which was 2.0 times higher than that (455 mW cm⁻²) of the cell with the configuration of Ni–YSZ/YSZ/LSM(La_0.8Sr_0.2MnO_3−δ)–YSZ/LSM at 750 °C.     

High-performance cathode-supported SOFCs prepared by a single-step co-firing process

Cathode-supported solid oxide fuel cells (SOFCs), comprising porous Pr_0.35Nd_0.35Sr_0.3MnO_3−δ (PNSM)/Sm_0.2Ce_0.8O_1.95 (SDC) cathode supports, SDC function layers, YSZ electrolyte membranes and NiO/SDC anode layers, were successfully fabricated via suspensions coating and single-step co-firing process. The microstructures of electrolyte membranes were observed with scanning electron microscope (SEM). The assembled single cell was electrochemically characterized with humidified hydrogen as fuel and ambient air as oxidant. The open circuit voltage (OCV) of the cell was 1.036 V at 650 °C, and the peak power densities were 657, 472, 290 and 166 mW cm⁻² at 800, 750, 700 and 650 °C, respectively. Impedance analysis indicated that the performance of cathode-supported cell was determined essentially by electrode polarization resistance, which suggested that optimizing electrodes was especially important for improving the cell performance.     

YSZ films fabricated by a spin smoothing technique and its application in solid oxide fuel cell

A dense single-layer YSZ film has been successfully fabricated by a spin smoothing method. Followed by a simplified slurry coating, an additional spin smoothing process was conducted to obtain a thinner and smoother film. By employment of high-viscosity slurry including high YSZ content, the film has a suitable thickness by a single coating cycle. With Sm_0.2Ce_0.8O_1.9 (SDC)-impregnated La_0.7Sr_0.3MnO₃ (LSM) cathode and porous NiO–YSZ anode, single solid oxide fuel cell (SOFC) based on an 8-μm-thick YSZ film was obtained. Open-circuit voltage (OCV) of the cell was 1.04 V at 800 °C, and maximum power densities were 676, 965 and 1420 mW cm⁻² at 700, 750 and 800 °C, respectively, using H₂ at a flow rate of 40 mL min⁻¹ as fuel and ambient air as oxidant. The power density could be increased to 1648 mW cm⁻² at 800 °C when the flow rate of H₂ was enhanced to 200 mL min⁻¹.     

Plasma sprayed metal supported YSZ/Ni–LSGM–LSCF ITSOFC with nanostructured anode

Intermediate temperature solid oxide fuel cells (ITSOFCs) supported by a porous Ni-substrate and based on Sr and Mg doped lanthanum gallate (LSGM) electrolyte, lanthanum strontium cobalt ferrite (LSCF) cathode and nanostructured yttria stabilized zirconia–nickel (YSZ/Ni) cermet anode have been fabricated successfully by atmospheric plasma spraying (APS). From ac impedance analysis, the sprayed YSZ/Ni cermet anode with a novel nanostructure and advantageous triple phase boundaries after hydrogen reduction has a low resistance. It shows a good electrocatalytic activity for hydrogen oxidation reactions. The sprayed LSGM electrolyte with ∼60 μm in thickness and ∼0.054 S cm⁻¹ conductivity at 800 °C shows a good gas tightness and gives an open circuit voltage (OCV) larger than 1 V. The sprayed LSCF cathode with ∼30 μm in thickness and ∼30% porosity has a minimum resistance after being heated at 1000 °C for 2 h. This cathode keeps right phase structure and good porous network microstructure for conducting electrons and negative oxygen ions. The APS sprayed cell after being heated at 1000 °C for 2 h has a minimum inherent resistance and achieves output power densities of ∼440 mW cm⁻² at 800 °C, ∼275 mW cm⁻² at 750 °C and ∼170 mW cm⁻² at 700 °C. Results from SEM, XRD, ac impedance analysis and I–V–P measurements are presented here.     

Direct ceramic inkjet printing of yttria-stabilized zirconia electrolyte layers for anode-supported solid oxide fuel cells

Electromagnetic drop-on-demand direct ceramic inkjet printing (EM/DCIJP) was employed to fabricate dense yttria-stabilized zirconia (YSZ) electrolyte layers on a porous NiO-YSZ anode support from ceramic suspensions. Printing parameters including pressure, nozzle opening time and droplet overlapping were studied in order to optimize the surface quality of the YSZ coating. It was found that moderate overlapping and multiple coatings produce the desired membrane quality. A single fuel cell with a NiO-YSZ/YSZ (∼6 μm)/LSM + YSZ/LSM architecture was successfully prepared. The cell was tested using humidified hydrogen as the fuel and ambient air as the oxidant. The cell provided a power density of 170 mW cm⁻² at 800 °C. Scanning electron microscopy (SEM) revealed a highly coherent dense YSZ electrolyte layer with no open porosity. These results suggest that the EM/DCIJP inkjet printing technique can be successfully implemented to fabricate electrolyte coatings for SOFC thinner than 10 μm and comparable in quality to those fabricated by more conventional ceramic processing methods.     

Study of slurry spin coating technique parameters for the fabrication of anode-supported YSZ Films for SOFCs

A slurry spin coating method was developed to fabricate gas-tight anode-supported YSZ films for solid oxide fuel cells (SOFCs). Several technique parameters for slurry spin coating, such as the slurry viscosity, spinning speed, number of coating cycles, film thickness and their effects on YSZ electrolyte film were investigated. SEM results, open-circuit voltage (OCV) values and cell performance indicated that these parameters had crucial and obvious influences on YSZ film quality and fuel cell performance. Based on the optimized parameters, anode-supported YSZ films and several single fuel cells were successfully fabricated and tested. An OCV as high as 1.06 V was obtained at 800 °C and maximum power densities of 900, 1567, 2005 mW cm⁻² were achieved at 700, 750, 800 °C, respectively, using hydrogen as fuel and ambient air as oxidant.     

A high-performance ammonia-fueled SOFC based on a YSZ thin-film electrolyte

Ammonia is a possible candidate as the fuel for solid oxide fuel cells (SOFCs). In this work, an anode-supported SOFC based on yttrium-stabled zircite (YSZ) thin-film electrolyte was fabricated by a simple dry-pressing process. Directly fueled by commercial liquefied ammonia, the single cell was tested at temperatures from 650 to 850 °C. The maximum power densities were 299 and 526 mW cm⁻² at 750 and 850 °C, respectively, only slightly lower than that fueled by hydrogen. Analysis of open current voltages (OCVs) of the cell indicated the oxidation of ammonia within a SOFC is a two-stage process. Impedance spectra showed the cell fueled by ammonia had the same electrolyte resistances as that fueled by hydrogen, but a little larger interfacial polarization resistances. Further, the performances of the cell were essentially determined by the interfacial resistances under 750 °C.     

Performance and degradation of metal-supported solid oxide fuel cells with impregnated electrodes

Metal-supported solid oxide fuel cells (MS-SOFCs) containing porous 430L stainless steel supports, YSZ electrolytes and porous YSZ cathode backbones are fabricated by tape casting, laminating and co-firing in a reducing atmosphere. Nano-scale Ni and La_0.6Sr_0.4Fe_0.9Sc_0.1O_3−δ (LSFSc) coatings are impregnated onto the internal surfaces of porous 430L and YSZ, acting as the anode and the cathode catalysts, respectively. The resulting MS-SOFCs exhibit maximum power densities of 193, 418, 636 and 907 mW cm⁻² at 650, 700, 750 and 800 °C, respectively. Nevertheless, a continuous degradation in the fuel cell performance is observed at 650 °C and 0.7 V during a 200-h durability measurement. Possible degradation mechanisms were discussed in detail.     

Fabrication and characterization of a Ba0.5Sr0.5Co0.8Fe0.2O3−δ—Gadolinia-doped ceria cathode for an anode-supported solid-oxide fuel cell

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) and gadolinia-doped ceria (GDC) were synthesized via a glycine-nitrate process (GNP). A cubic perovskite of BSCF was observed by X-ray diffraction (XRD) at a calcination temperature above 950 °C. An anode-supported solid-oxide fuel cell was constructed from the porous NiO + YSZ as the anode substrate, the yittria-stabilized zirconia (YSZ) as the electrolyte, and the porous BSCF-GDC layer as the cathode with a GDC barrier layer. For the performance test, the maximum power density was 191.3 mW cm⁻² at a temperature of 750 °C with H₂ fuel and air at flow rates of 335 and 670 sccm, respectively. According to the AC-impedance data, the charge-transfer resistances of the electrodes were 0.10 and 1.59 Ω cm², and the oxygen-reduction and oxygen-ion diffusion resistances were 0.69 and 0.98 Ω cm² at 750 and 600 °C, respectively. SEM microstructural characterization indicated that the fuel cell as fabricated exhibited good compatibility between cathode and electrolyte layers.     

Performance and durability of nanostructured (La0.85Sr0.15)0.98MnO3/yttria-stabilized zirconia cathodes for intermediate-temperature solid oxide fuel cells

In this communication we report the fabrication of nanostructured (La_0.85Sr_0.15)_0.98MnO₃ (LSM)/yttria-stabilized zirconia (YSZ) composite cathodes consisting of homogeneously distributed and connected LSM and YSZ grains approximately 100 nm large. We also investigate for the first time the role of the cathode nanostructure on the performance and the durability of intermediate-temperature solid oxide fuel cells. The cathodes were fabricated using homogenous LSM/YSZ nanocomposite particles synthesized by co-precipitation, using YSZ nanoparticles of 3 nm as seed crystals. Detailed microstructural characterization by transmission electron microscopy with energy-dispersive X-ray spectroscopy revealed that many of the LSM/YSZ junctions in the cathode faced the homogeneously connected pore channels, indicating the formation of a considerable number of triple phase boundaries. The nanostructure served to reduce cathodic polarization. As a result, these anode-supported solid oxide fuel cells showed high power densities of 0.18, 0.40, 0.70 and 0.86 W cm⁻² at 650, 700, 750 and 800 °C, respectively, under the cell voltage of 0.7 V. Furthermore, no significant performance degradation of the cathode was observed during operation at 700 °C for 1000 h under a constant current density of 0.2 A cm⁻².     

Vertically aligned nanocomposite La0.8Sr0.2MnO3−δ/Zr0.92Y0.08O1.96 thin films as electrode/electrolyte interfacial layer for solid oxide reversible fuel cells

A thin layer with a vertically aligned nanocomposite (VAN) structure of La_0.8Sr_0.2MnO_3−δ (LSM) and Zr_0.92Y_0.08O_1.96 (YSZ) between the oxygen electrode and the electrolyte has been fabricated by a pulsed laser deposition (PLD) technique for solid oxide reversible fuel cells (SORFCs). The high quality epitaxial growth of VAN structured LSM/YSZ has been achieved on single crystal SrTiO₃ substrate at high-deposition temperatures. The symmetric cells with the VAN interlayer are found to have a lower area specific resistance compared to that without the interlayer. The enhancement in performance has been demonstrated by increased oxygen electrode catalytic properties and porous oxygen electrode microstructure. The cell with the VAN interlayer shows an open circuit voltage (OCV) of 1.00 V at 650 °C and maximum power densities of 0.22, 0.32, 0.43 and 0.55 W cm⁻² at 650, 700, 750 and 800 °C, respectively. Compared with the cell without an interlayer, the cells with the interlayer have ∼2 times of the overall maximum power density at the measured temperature range, demonstrating that the VAN interlayer significantly enhances the oxygen electrode performance.     

Fabrication and characterization of Sm0.2Ce0.8O2−δ-Sm0.5Sr0.5CoO3−δ composite cathode for anode supported solid oxide fuel cell

The Sm_0.5Sr_0.5CoO_3−δ (SSC) with perovskite structure is synthesized by the glycine nitrate process (GNP). The phase evolution of SSC powder with different calcination temperatures is investigated by X-ray diffraction and thermogravimetric analyses. The XRD results show that the single perovskite phase of the SSC is completely formed above 1100 °C. The anode-supported single cell is constructed with a porous Ni-yttria-stabilized zirconia (YSZ) anode substrate, an airtight YSZ electrolyte, a Sm_0.2Ce_0.8O_2−δ (SDC) barrier layer, and a screen-printed SSC-SDC composite cathode. The SEM results show that the dense YSZ electrolyte layer exhibits the good interfacial contact with both the Ni-YSZ and the SDC barrier layer. The porous SSC-SDC cathode shows an excellent adhesion with the SDC barrier layer. For the performance test, the maximum power densities are 464, 351 and 243 mW cm⁻² at 800, 750 and 700 °C, respectively. According to the results of the electrochemical impedance spectroscopy (EIS), the charge-transfer resistances of the electrodes are 0.49 and 1.24 Ω cm², and the non charge-transfer resistances are 0.48 and 0.51 Ω cm² at 800 and 700 °C, respectively. The cathode material of SSC is compatible with the YSZ electrolyte via a delicate scheme employed in the fabrication process of unit cell.     

The effect of porosity gradient in a Nickel/Yttria Stabilized Zirconia anode for an anode-supported planar solid oxide fuel cell

In this paper, a graded Ni/YSZ cermet anode, an 8 mol.%YSZ electrolyte, and a lanthanum strontium manganite (LSM) cathode were used to fabricate a solid oxide fuel cell (SOFC) unit. An anode-supported cell was prepared using a tape casting technique followed by hot pressing lamination and a single step co-firing process, allowing for the creation of a thin layer of dense electrolyte on a porous anode support. To reduce activation and concentration overpotential in the unit cell, a porosity gradient was developed in the anode using different percentages of pore former to a number of different tape-slurries, followed by tape casting and lamination of the tapes. The unit cell demonstrated that a concentration distribution of porosity in the anode increases the power in the unit cell from 76 mW cm⁻² to 101 mW cm⁻² at 600 °C in humidified hydrogen. Although the results have not been optimized for good performance, the effect of the porosity gradient is quite apparent and has potential in developing superior anode systems.     

Enhanced performance of a single-chamber solid oxide fuel cell with an SDC-impregnated cathode

The electrochemical performance of anode-supported single-chamber solid oxide fuel cells (SC-SOFCs) with and without SDC-impregnated cathodes was compared in a diluted methane–oxygen mixture. These cells were made of conventional materials including yttrium-stabilized zirconia (YSZ) thin film, a Ni + YSZ anode and a La_0.7Sr_0.3MnO₃ (LSM) cathode. Our results showed that the cell performance was greatly enhanced with the SDC-impregnated LSM cathode. At a furnace temperature of 750 °C, the maximum power density was as high as 404 mW cm⁻² for a CH₄ to O₂ ratio of 2:1, which was 4.0 times higher than the cell with a pure LSM cathode (100 mW cm⁻²). The overall polarization resistance of the impregnated cell was 1.6 Ω cm², which was much smaller than that of the non-impregnated one (4.2 Ω cm²). The impregnation introduced SDC nanoparticles greatly extended the electrochemical active zone and hence greatly improved the cell performance.     

Metal-supported solid oxide fuel cells with in-situ sintered (Bi2O3)0.7(Er2O3)0.3–Ag composite cathode

Metal-supported solid oxide fuel cells (SOFCs) containing porous 430L stainless steel support, Ni-YSZ anode and YSZ electrolyte were fabricated by tape casting, laminating and co-firing in a reduced atmosphere. (Bi₂O₃)_0.7(Er₂O₃)_0.3–Ag composite cathode was applied by screen printing and in-situ sintering. The polarization resistances of the composite cathode were 1.18, 0.48, 0.18, 0.09 Ω cm² at 600, 650, 700 and 750 °C, respectively. A promissing maximum power density of 568 mW cm⁻² at 750 °C was obtained of the single cell. Short-term stability was measured as well.     

Development of LSM-based cathodes for solid oxide fuel cells based on YSZ films

In an attempt to achieve desirable cell performance, the effects of La_0.7Sr_0.3MnO₃ (LSM)-based cathodes on the anode-supported solid oxide fuel cells (SOFCs) were investigated in the present study. Three types of cathodes were fabricated on the anode-supported yttria-stabilized zirconia (YSZ) thin films to constitute several single cells, i.e., pure LSM cathode, LSM/YSZ composite by solid mixing, LSM/Sm_0.2Ce_0.8O_1.9 (SDC) composite by the ion-impregnation process. Among the three single cells, the highest cell output performance 1.25 W cm⁻² at 800 °C, was achieved by the cell using LSM/SDC cathode when the cathode was exposed to the stationary air. Whereas, the most considerable cell performance of 2.32 W cm⁻² was derived from the cell with LSM/YSZ cathode, using 100 ml min⁻¹ oxygen flow as the oxidant. At reduced temperatures down to 700 °C, the LSM/SDC cathode was the most suitable cathode for zirconia-based electrolyte SOFC in the present study. The variation in the cell performances was attributed to the mutual effects between the gas diffusing rate and three-phase boundary length of the cathode.